Relative Aperture of Camera Lens

Relative Aperture of Camera Lens

Every lens has a focal length (or, in the
case of zoom lenses, a set range of focal lengths). Other than focal
length, every lens also has an aperture, which is the opening through
which the light -asses to form the image on the camera sensor or film.
Camera lenses (other than fixed-aperture mirror lenses) can be set at
any of a number of specific apertures.

The diameter of the opening is called the
relative aperture, and is identified by its f-number (to a degree, the
terms are used interchangeably by photographers). Relative aperture is
the mathematical ratio between the focal length of a lens and the
diameter of the aperture (dividing the focal length by the aperture
diameter yields the number). A lens whose aperture is open to a
diameter of one-eighth of its focal length is said to be set at f/8; if
open to one-quarter of its focal length it is set at f/4; and so on.

Along with the shutter-speed setting, the
setting of the lens aperture determines the amount of light passing
through the lens and thus the amount of exposure given to the camera
sensor or film. The size of the aperture is varied by means of an iris
diaphragm consisting of several movable metal leaves in an assembly
placed within the lens mount, usually between two of the glass lens
elements.

Lenses are classified for speed (the
maximum amount of light that can be passed through them in a given
time) and by the size of their maximum opening. Thus, a 50 mm
focal-length lens having a maximum opening of 25 mm has a maximum
relative aperture of f/2, and is described as an f/2 lens. A lens is
said to be fast if its maximum aperture is relatively large - f/2.8 or
larger. A slow lens would have a maximum aperture of f/6.3 or f/8 (such
lenses are frequently used with view cameras). Lenses falling between
these categories are, of course, called medium-speed lenses. Note that,
as the actual size of the aperture gets larger, the f-number describing
it becomes smaller, and vice versa, a consequence of the proportional
mathematics used.

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